Occlusive devices are placed within an opening or cavity in the body, such as in the vasculature, spinal column, fallopian tubes, bile ducts, bronchial and other air passageways and the like, and are generally delivered using minimally invasive surgical techniques. In general, an implantable device is guided to a desired site through a delivery catheter and may be pushed through an opening at the distal end of a delivery catheter by a pusher mechanism, such as a pusher or delivery wire, thereby deploying the device at the desired site. Once the occlusive device has been placed in the desired position, it is detached from the pusher mechanism without disturbing placement of the occlusive device or damaging surrounding structures.
Occlusive coils for implantation into anatomical defects such as aneurysms and other blood vessel abnormalities are well known. Aneurysms are bulges that form in an artery wall, generally caused by a weakening in the artery wall, that form an opening or cavity and are often the site of internal bleeding and stroke. In general, the minimally invasive therapeutic objective is to prevent material that collects or forms in the cavity from entering the bloodstream and to prevent blood from entering and collecting in the aneurysm. This is often accomplished by introducing various materials and devices into the aneurysm.
Various types of embolic agents and devices are used to reduce risks to a patient associated with the presence of an aneurysm. One class of embolic agents includes injectable fluids or suspensions, such as microfibrillar collagen, various polymeric beads, and polyvinylalcohol foam. These polymeric agents may be crosslinked (sometimes in vivo) to extend the persistence of the agent at the vascular site. These agents are often introduced into the vasculature through a catheter. After introduction and at the site, the introduced materials form a solid space-filling mass. Although some of these agents provide for excellent short term occlusion, many are thought to allow vessel recanalization due to absorption into the blood. Other materials, such as hog hair and suspensions of metal particles, have also been proposed and used to promote occlusions. Polymer resins, such as cyanoacrylates, are also employed as injectible vaso-occlusive materials. These resins are typically mixed with a radio-opaque contrast material or are made radio-opaque by the addition of a tantalum powder. Accurate and timely placement of these mixtures is crucial and very difficult. These materials are also difficult or impossible to retrieve once they have been placed in the vasculature.
Implantable vaso-occlusive metallic structures are also well known and commonly used. Many vaso-occlusive devices are provided in the configuration of helical coils and are constructed from a shape memory material that forms a desired coil configuration upon exiting the distal end of a delivery catheter. The purpose of the coil is to fill the space formed by a defect or injury and facilitate formation of an embolus with the associated allied tissue. Multiple coils of the same or different structure may be implanted serially in a single aneurysm or other vessel defect. Implantable framework structures are also used in an attempt to stabilize the wall of the aneurysm or defect prior to insertion of filling material such as coils.
Vaso-occlusive coils are generally constructed from metal or metal alloy wire forming a helical spiral. These devices may be formed from a shape change alloy and introduced to the target site through a catheter in a stretched linear form. The vaso-occlusive device assumes its predetermined, non-stretched three dimensional form upon discharge from the distal end of the catheter. Numerous coil and other three-dimensional structures are known in the art for occlusion of vascular abnormalities such as aneurysms.
Techniques for delivering a vaso-occlusive device to a target site generally involve a delivery catheter and a detachment mechanism that detaches the coil from a delivery mechanism after placement at the target site. A microcatheter is initially steered through the delivery catheter into or adjacent to the entrance of an aneurysm, typically aided by the use of a steerable guidewire. The guidewire is then withdrawn from the micro catheter lumen and replaced by the implantable vaso-occlusive coil. The vaso-occlusive coil is advanced through and out of the microcatheter and thus deposited within the aneurysm or other vessel abnormality. Implantation of the vaso-occlusive device within the internal volume of a cavity and maintenance of the device within the internal volume of the aneurysm is crucial. Migration or projection of a vaso-occlusive device from the cavity may interfere with blood flow or nearby physiological structures and poses a serious health risk.
One type of aneurysm, commonly known as a “wide neck aneurysm” is known to present particular difficulty in the placement and retention of vaso-occlusive coils. Wide neck aneurysms are generally referred to as aneurysms of vessel walls having a neck or an entrance zone from the adjacent vessel that is large compared to the diameter of the aneurysm or that is clinically observed to be too wide effectively to retain vaso-occlusive coils deployed using the techniques discussed above.
Devices for maintaining vaso-occlusive coils within an aneurysm have been proposed. One such device is described in U.S. Pat. No. 5,980,514, which discloses devices that are placed within the lumen of a feed vessel exterior to the aneurysm to retain coils within the aneurysm cavity. The device is held in place by means of radial pressure of the vessel wall. After the device is released and set in an appropriate place, a microcatheter is inserted into the lumen behind the retainer device and the distal end of the catheter is inserted into the aneurysm cavity for placement of one or more vaso-occlusive devices. The retainer device prevents migration of occlusion devices from the cavity.
Another methodology for closing an aneurysm is described in U.S. Pat. No. 5,749,894, in which a vaso-occlusive device such as a coil or braid has on its outer surface a polymeric composition that reforms or solidifies in situ to provide a barrier. The polymer may be activated, e.g. by the application of light, to melt or otherwise to reform the polymer exterior to the vaso-occlusive device. The vaso-occlusive device then sticks to itself at its various sites of contact and forms a rigid whole mass within the aneurysm.
Devices for bridging the neck of an aneurysm have been proposed. U.S. Patent Application 2003/0171739 A1, for example, discloses a neck bridge having one or more array elements attached to a junction region and a cover attached to the junction region and/or the array elements. The array elements may comprise Nitinol loops and the cover may comprise a fabric, mesh or other sheeting structure.
The placement of coils or other structures or materials in the internal space of an aneurysm or other defect hasn't been entirely successful. The placement procedure may be arduous and lengthy, requiring the placement of multiple coils serially in the internal space of the aneurysm. Debris and occlusive material may escape from within the aneurysm and present a risk of stroke, vessel blockage or other undesirable complications. Blood flows into aneurysm and other blood vessel irregularities after the placement of embolic devices, which increases the risks of complication. Furthermore, some aneurysms, vessels and other passageway defects aren't well-suited to placement of coils or other conventional occlusive devices. Wide neck aneurysms continue to present challenges in the placement and retention of vaso-occlusive coils.
Vaso-occlusive coils may be classified based upon their delivery mechanisms as pushable coils, mechanically detachable coils, and electrolytically detachable coils. Pushable coils are commonly provided in a cartridge and are pushed or “plunged” from the cartridge into a delivery catheter lumen. A pusher advances the pushable coil through and out of the delivery catheter lumen and into the site for occlusion. Mechanically detachable vasoocclusive devices are typically integrated with a pusher rod and are mechanically detached from the distal end of that pusher after exiting a delivery catheter. Electrolytically detachable vaso-occlusive devices are generally attached to a pusher by means of an electrolytically severable joint. The electrolytic joint may be severed by the placement of an appropriate voltage on the core wire. The joint erodes in preference either to the vaso-occlusive device itself or to the pusher core wire. The core wire is often simply insulated to prevent the electrolytic response caused by the imposition of electrical current.
Systems currently known, more generally, for the detachment of implantable devices after placement include mechanical systems, electrolytic systems and hydraulic systems. In mechanical systems, the occlusive device and the pusher wire are linked by means of a mechanical joint, or inter-locking linkage, which separates once the device exits the delivery catheter, thereby releasing the device. Examples of such systems include those taught in U.S. Pat. Nos. 5,263,964, 5,304,195, 5,350,397, and 5,261,916.
In electrolytic systems, a constructed joint (generally either fiber- or glue-based) connects the pusher wire to the occlusive device. Once the device has been placed in the desired position, the joint is electrolytically disintegrated by the application of a current or heat (for example, using a laser) by the physician. An example of such a system is provided in U.S. Pat. No. 5,624,449. Such systems have the disadvantage that dissolved material or gases generated by electrolysis may be released into the vasculature, thus presenting a potential hazard to the patient. Electrolytic detachment may also take more time to accomplish than is desirable during an interventional operation in which several occlusive devices are placed.
In hydraulic systems, the pushing wire is connected to the occlusive device by means of a polymer coupling. The pushing wire contains a micro-lumen to which the physician attaches a hydraulic syringe at the proximal end of the pusher wire. Upon the application of pressure on the syringe plunger, the hydraulic pressure increases and forces the polymer joint to swell and break, thereby releasing the device. An example of a hydraulic system is that described in U.S. Pat. No. 6,689,141.
U.S. Pat. No. 5,911,737 discloses a mechanism for releasing an implantable device that utilizes shape memory polymer microtubing that is heated above its phase transformation temperature, then shaped to hold a portion of the device to be implanted, and then cooled so that the device is retained by the tubing. Once the device has been positioned in the desired location, the microtubing is heated above its phase transformation temperature, thereby releasing the device.
Shape memory material is material that exhibits mechanical memory when activated by heat. Shape memory alloys have a transition temperature that depends upon the particular ratio of the metals in the alloy. Such alloys can be formed into a first shape when heated to a temperature sufficient for the material to reach its austenitic phase, and then plastically deformed into a second shape upon being brought below the transition temperature (martensitic state). When the alloy is reheated above its transition temperature, the alloy transforms from its martensitic phase to its austenitic phase and returns to its first, pre-set, shape. Shape memory alloys have been employed in implantable devices, as described, for example, in U.S. Pat. No. 3,868,956.
U.S. Pat. No. 5,578,074 discloses an implant delivery system which includes a pusher having a coupling portion formed of shape memory material which exhibits different configurations depending on the temperature. The coupling portion interlockingly engages the implant when it is in a generally bent or coiled configuration and releases the implant when thermally activated to assume its pre-set configuration. US Published Patent Application No. 2003/0009177 describes apparatus for manipulating matter, such as an implantable device within a body lumen, comprising a manipulator means constructed of one or more bent or twisted shape memory alloy (SMA) members having pseudoelasticity at body temperature, and a hollow housing capable of holding the SMA member in a relatively straightened state. On being extended from the housing at body temperature, the shape memory alloy member bends or twists in a lateral or helical direction in order to manipulate the device. On being withdrawn into the housing, the shape memory alloy member becomes relatively straightened.
Despite the variety of detachment mechanisms available, there is a continuing need for reliable, fast-acting mechanisms for detaching implantable devices that do not release material upon detachment and do not interfere with device placement or the surrounding physiological structures. The detachment mechanisms described herein are directed to meeting this need.